The invention relates to a variable power transmission apparatus in which rotary power of one shaft is transferred to another shaft, with a corresponding change in speed and torque.
Many different types of variable power transmissions are available; some types using trains of journalled gears which cooperate with each other so that speed and torque applied to an input shaft are changed at an output shaft. In some applications, such as a pedal bicycle drive means, a chain loop extends between an input chain wheel, and an output sprocket driving a bicycle wheel. A common form of varying ratio between the chain wheel and output sprocket is to provide a series of sprockets of different sizes, and to shift the chain between the sprockets. A resiliently mounted "jockey wheel" accommodates variations in chain "slackness". This produces incremental changes in output torque and speed of rotation for each sprocket, i.e. there are increments or spaces between adjacent gear settings, but if a wide gear range is required, the increments between adjacent gears tend to be large. If the number of gear settings is to be increased, so as to decrease the spacing between adjacent gear settings, additional complexities are introduced such as multiple chain wheels. Multiple chain wheels usually produce "overlapping" or duplicated gear settings, which reduce effectiveness of the prior art. Furthermore the gear changing mechanism is relatively fragile, and difficult to protect and thus is relatively prone to impact damage. Furthermore, components of the gear changing mechanism are usually exposed to contamination, and it can be difficult to provide adequate lubrication and protection from the elements.
Both types of transmissions described above have distinct increments or "steps" between each gear setting, i.e. they do not provide a continuously variable speed or torque throughout the entire range between upper and lower limits of the gear transmission.
While attempts have been made to produce "stepless" transmissions, that is a transmission apparatus in which speed and torque varies continuously over the entire range, additional complications are usually introduced. While stepless transmissions have the advantage of continuous variation in torque and speed between the input and output limits, usually the range of gear ratios is relatively limited compared with the conventional "stepped" transmission as previously described.
On example of a stepless transmission is shown in U.S. Pat. No 4,411,165 issued to Evans in 1983. While this provides a stepless transmission which can be designed to have a relatively wide gear ratio range, mechanical complexity is considerable and its application to a pedal bicycle would be costly.